Muscle action potentials and miniature end-plate potential frequency were studied in different concentrations of pindolol, d- and l-propranolol and sotalol using intracellular microelectrode recording from rat phrenic nerve-diaphragm preparations. d-and l-propranolol at concentrations of 10 to 40 mg/l and pindolol at 20 to 100 mg/l slowed down the rate of rise, prolonged rise and half-decay times and reduced the overshoot of the muscle action potentials. Sotalol had similar effects but only at higher concentrations (100 to 200 mg/l). The drugs had no effect on the increase in the miniature end-plate potential frequence obtained with depolarization by increased extracellular potassium concentration.